Why do some toilet bowls have stains inside of them? Quite often the staining that appears around the water line and/or under the inside rim is caused by hard water with a high amount of minerals in it. Water evaporates and when it does white calcium, rust colored iron compounds, or magnesium compounds create stains. Worn-out bowl surfaces tend to stain rapidly versus polished bowl surfaces are more resistant to stains.
Different cleaning solutions and techniques are available to combat the staining problem. There are in-tank cleaners that appear to remove the stains but they actually mask the stains. Non-acidic cleaners work well on non-mineral stains but are much less effective on mineral stains. High acidity cleaning solutions used in conjunction with a brush cleaning is effective and create the fastest results. Powdered cleaners are difficult to use under the toilet bowl rims compared to liquid cleaners.
Toilet bowl cleaners are made to work and are powerful, so be sure to handle them carefully and never mix them with other types of cleaners. Mixing the cleaners could release unsafe toxic fumes.